1. Technical Field
The present invention relates to a multiple-phase rectifying apparatus for use in an electric supply system comprising two or more phases, said apparatus comprising a single phase rectifier having a first and a second primary terminal, for each phase, each rectifier connected by its first primary terminal to the respective phase, the second primary terminals of said rectifiers being interconnected.
2. Description of Related Art
Such networks, and such rectifiers are generally known in the art of electric supply systems.
A rectifier for a three phase network may be designed using three single phase rectifiers connected in a star configuration. For example, for a 3xc3x97400V network, three 230V rectifiers may be used.
If there is no available neutral point in the three phase network, the neutral point created by the three rectifiers in a star configuration may be unstable. Asymmetric effects between the phases may occur both in the stationary and the transient state. Oscillations may be induced in rectifiers that are normally stable, when supplied from a low impedance network. This is because each rectifier xe2x80x9cseesxe2x80x9d the mains through two other rectifiers making it a high impedance network.
It is therefore an object of the invention to provide a stable artificial neutral point in a network.
This object is achieved according to the invention by a multiple-phase rectifying apparatus as initially defined, characterized in that it comprises means connectable in parallel with said single-phase rectifiers for creating a neutral point in the electric supply system.
By creating a neutral point in the system, the connection of three single-phase rectifiers in a star configuration with phase voltages that are as equal as possible is enabled.
Also, the creation of a neutral point independently of the neutral point of the supply network, especially in cases where the neutral potential is not located in the middle of the phase vector system is enabled.
The means for creating a neutral point is not connected to the neutral conductor of the electric system.
The means for creating a neutral point preferably includes magnetic components of such a kind that the vector sum of the phase voltages is zero. Preferably, said means for creating a neutral point is arranged to short-circuit the zero sequence component of the phase voltages without affecting the positive and negative sequence components.
According to a first preferred embodiment the means for creating a neutral point comprises one transformer for each phase, the primary winding of each transformer connected to the respective phase in a star configuration, the secondary windings serially connected and the interconnection point of the primary windings forming said neutral point.
According to a second preferred embodiment the means for creating a neutral point comprises one winding for each phase, each winding wound on one leg of a common three-legged core, one end of each winding connected to the respective phase and the other end of each winding being interconnected, the interconnection point of the windings forming said neutral point.
According to a third preferred embodiment the means for creating a neutral point comprises a first, a second and a third transformer, each transformer comprising a first and a second winding, and each phase being connected to the artificial neutral point through the first winding of one transformer and the second winding of another transformer in the opposite direction from said first winding, in a zigzag coupling.
This third preferred embodiment has the advantage that since no secondary windings are used, the size of the component may be reduced, or the cross section of the conductor used in the winding may be increased, thereby reducing the impedance of the transformer, without increasing the size of the component.
The means for creating a neutral point according to the invention may also be used in a supply system in which the neutral point is not found in the centre of the phase diagram of the system, for example a supply system used in Japan.
The means creates a neutral point to which the zero sequence component of the phase voltages has a low impedance connection while the positive sequence and negative sequence components have high impedance connections.
Connecting the means for creating a neutral point does not affect positive and negative sequence voltage components, that is, it does not affect the supply system, which constitutes a positive sequence system.
The invention requires the addition of magnetic components of the order of magnitude of 5% of the power of the rectifier.
The invention offers the following advantages:
Any kind of single phase rectifier may be used, thus, a stable three-phase rectifier can be achieved at a relatively low cost.
The rectifier according to the invention can be used both for two and three phases.
The artificial neutral point achieved in this way does not cause any additional load in the supply system.
The zero sequence component of the phase voltages is short-circuited.
The third harmonic component and its multiples are short-circuited.
Other advantages include the possibility to change from a 3xc3x97400V network to a 3xc3x97230V network using the same single-phase rectifiers, and to operate when one phase is missing.